When we consume a food or drink, the nutrients contained are released from the matrix, absorbed by the blood and transported to the respective tissues. However, not all nutrients can be used equally. In other words, they differ in their bioavailability. Understanding the bioavailability of a nutrient helps optimize diets and establish appropriate nutrient supplies.
Define the bioavailability of a nutrient
There are several definitions of the bioavailability of a nutrient, but broadly it refers to the proportion of a nutrient that is absorbed from the diet and used for normal body functions.1,2 The following components describe the different steps of the metabolic pathway in which Changes in the bioavailability of a nutrient 1 can occur:
- release of the nutrient from the physico-chemical food matrix
- effects of digestive enzymes in the intestine
- binding and absorption by the intestinal mucosa
- transfer through the intestinal wall (through the cells, within-between them or both) into the blood or lymphatic system
- systemic distribution
- systemic deposition (conservation)
- metabolic and functional use
- excretion (via urine or faeces)
As is evident from this list, the bioavailability of a nutrient is regulated by internal and external factors. External factors include the food matrix and chemical form of the nutrient in question, while gender, age, nutrient status and life stage (e.g. pregnancy) are among the internal factors. Since aspects such as nutrient status also determine whether and how much of a nutrient is actually used, stored or excreted, some definitions of bioavailability are limited to the fraction of a nutrient that is absorbed.3
The bioavailability of macronutrients – carbohydrates, proteins, fats – is usually very high up to over 90% of the amount ingested. On the other hand, micronutrients, i.e. vitamins and minerals and bioactive phytochemicals (e.g. flavonoids, carotenoids) can vary widely in how they are absorbed and used. Therefore, the following sections will use micronutrients and phytochemicals as examples to illustrate the different stages in which the bioavailability of a nutrient can be affected.
Effects of the food matrix and the chemical form of nutrients
The first step in making a nutrient bioavailable is to free it from the food matrix and transform it into a chemical form that can bind and enter or pass between intestinal cells. This is generally referred to as bioaccessibility.4 Nutrients are made bioaccessible by chewing processes and the initial enzymatic digestion of the food in the mouth, along with the acid and other enzymes in the gastric juice after swallowing and finally released into the small intestine, the main site of nutrient absorption. Here, even more enzymes, supplied by the pancreatic juice, continue to degrade the food matrix.
In addition to the means of chewing and enzymatic activity of the organism, the digestibility of food matrices, especially vegetables, is favored by cooking or by homogenizing the food. For example, while raw carrots and spinach are a good source of dietary fiber, cooking them allows the human body to extract even a larger fraction of the carotenoids contained.
Minerals and other nutrients exist in different chemical forms in the food and this can affect their bioavailability. A classic example is iron. In general, we are talking about about two types of dietary iron; heme and non-heme iron. The former is found only in meat, fish and poultry, while the latter is found in foods of animal and plant origin. Heme iron is derived from the hemoglobin and myoglobin molecules responsible for oxygen transport and storage in the blood and muscles, respectively. Once released from the food matrix, the heme molecule acts as a protective ring around the central iron atom. In doing so, it protects iron from interacting with other food components, it keeps it soluble in the intestine and is absorbed intact through a specific transport system on the surface of the intestinal cells. In contrast, non-heme iron is poorly soluble in the intestine and easily influenced by other components of the diet.2 Therefore only a small part is absorbed by the cells.
Sometimes vitamins and minerals are added to foods to increase their nutritional value – a process called fortification. In the case of vitamin B or folic acid, which is often added to breakfast cereals, flour and certain varieties, this added folic acid is usually more bioavailable than that naturally present in food, commonly referred to as dietary folate. Studies report 20-70% lower bioavailability of dietary folate (from fruit, vegetables or liver) compared to synthetic folic acid.7 This does not, however, mean that only foods fortified with folic acid should be consumed.
Promoters of the bioavailability of nutrients
Nutrients can interact with each other or with other dietary components at the absorption site, causing both a change in bioavailability and – if the promoters and inhibitors eliminate each other – a nil effect. Promoters can act in different ways such as maintaining a soluble nutrient or protecting it from interacting with inhibitors. For example, since carotenoids are fat-soluble, adding small amounts of fat or oil to a meal (3-5g per meal) improves their bioavailability.9 Similarly, meat, fish and poultry, while they themselves contain iron highly bioavailable, they are also known to increase iron absorption from all foods. Although this ‘meat factor’ has yet to be identified,
Vitamin C is also a powerful ‘helper’, being able to increase iron absorption by two to three times.11 This means, for example, that drinking a glass of orange juice with a cup of cereal helps the body to use a greater quantity of the iron contained in cereals.
Impact of inhibitors on nutrient bioavailability
Inhibitors can reduce the bioavailability of nutrients: i) by binding the nutrient in question in a form not recognized by the absorption systems on the surface of intestinal cells, ii) making the nutrient insoluble and therefore unavailable for absorption, iii) competing for the same absorption system. Phytic acid is very abundant in certain plant foods (e.g. legumes, whole grains, seeds, hazelnuts) and binds strongly to minerals such as calcium, iron and zinc in soluble and insoluble complexes which are unavailable for the Absorption.12 Ways to reduce the phytic acid content of foods include fermentation (e.g. extended leavening of wholemeal bread) or soaking and germinating legumes.
An example of competition for the same absorption system is the interaction between calcium and non-heme iron. Both minerals bind to a transporter on the surface of absorbent intestinal cells, but while non-heme iron enters cells in this way, calcium stops at the entrance and blocks further iron passage. This effect is most prominent when calcium or iron supplements are used externally to meal preparation.14 Therefore, the best advice is to use those supplements at different times of the day to avoid this interference.
The inhibitory effect of food constituents can also be used advantageously, as is the case with phytosterols. These natural compounds are extracted from certain plants and added in higher doses (about 2g per serving) to various other foods (e.g. fermented milk drinks) in order to lower the absorption of cholesterol, whether of origin food or produced by the human body 15
Internal or individual factors can be divided into gastrointestinal or systemic factors. The role of gastrointestinal factors is illustrated by the pathway of absorption of vitamin B12. This vitamin requires gastric acid to be released from the food matrix and undergo a series of bonds to the R protein, the intrinsic factor (IF) protein and finally the absorption of the IF-vitamin B12 complex in the small intestine.16 Protein R, IF and gastric acid are all produced in the gastric mucosa and the functional decline of this mucosa – as can occur in old age or in some conditions – can compromise their production and thus the bioavailability of vitamin B12.
Systemic factors include a lack of a certain nutrient or changes in psychological status, e.g. pregnancy. In both cases, the body can respond by increasing the respective path of nutrient absorption or utilization to meet the increased demand.14 Calcium and zinc are among the nutrients regulated in this way. On the other hand, some inflammatory conditions or infections can reduce the absorption capacity of the intestine. For example, iron absorption is down regulated in people with acute infections such as the common flu. 17
Impact on nutrient supply
For several nutrients – especially calcium, magnesium, iron, zinc, folate and vitamin A – knowledge of their bioavailability is necessary to translate physiological intake into actual dietary intake.14 The magnitude of adjustments varies according to nutrients, habitual diet. and a number of subject-related factors, most of which are difficult to establish. Considering all these influences, it is not surprising that nutrient-based dietary intakes vary between countries and institutions, but the EURRECA Network of Excellence strives to standardize assessment methodologies across Europe.18